Today’s web and mobile applications rely on application programming interfaces (APIs) and web services (web services) to exchange data and integrate with other systems. However, with the growing use of these technologies also comes new security challenges. Improperly secured APIs can become the target of attacks, leading to data leakage, unauthorized access or deletion of resources.

To effectively protect APIs and web services, organizations should conduct regular security tests to identify and eliminate potential vulnerabilities. These tests include analyzing authentication and authorization mechanisms, verifying correct implementation of communication protocols, and assessing resistance to attacks such as SQL Injection, Cross-Site Scripting (XSS) and Cross-Site Request Forgery (CSRF), among others.

In this article, we will introduce key aspects of API and web services security, discuss methods for testing their resilience to threats, and present best practices for protecting data and ensuring the integrity of IT systems.

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Why have APIs, though often invisible to the user, become a favorite target of cybercriminals?
What are the most common and insidious vulnerabilities lurking in your APIs according to the OWASP API Security Top 10?
What are specialized API penetration tests and what unique techniques do ethical hackers use to test them?
What fundamental principles of designing and implementing secure APIs (security by design) do you need to implement in your organization?
How does nFlo help uncover and secure the “invisible door” of your APIs, ensuring the integrity and confidentiality of your data?
Key findings: Security of APIs and Web Services

Why have APIs, though often invisible to the user, become a favorite target of cybercriminals?

In today’s hyper-connected digital world, application programming interfaces (APIs) and web services (Web Services) have become the silent heroes driving countless processes. They are the ones, like an intricate network of invisible bridges and corridors, that enable communication and data exchange between different applications, systems, mobile devices and cloud platforms. They allow your mobile app to pull data from a server, your online store to integrate with payment systems, and your internal business systems to exchange information in real time. But this crucial role, combined with the fact that APIs often operate on highly sensitive data and business logic, has made them an extremely attractive, yet often underestimated, target for cybercriminals.

The problem with API security stems from several fundamental factors. First, unlike traditional user interface web applications, APIs are inherently “headless” and designed for machine-to-machine communication. This means that many traditional protection mechanisms focused on human interaction (e.g., some types of WAF, bot protection based on user behavior) may be less effective in the context of APIs. Attackers can directly send crafted requests to API endpoints, often bypassing the first line of defense.

Second, the explosion in the number and complexity of APIs in recent years has caused many organizations to lose full visibility and control over their “API attack surface.” New APIs are quickly created and deployed by different development teams, often without consistent security standards or adequate oversight. Documentation is sometimes incomplete, and “forgotten” or undocumented endpoints (so-called shadow APIs or zombie APIs) can be easy targets for attackers who meticulously seek them out.

Third, APIs often have direct access to critical business data and functionality. Compromising an API can lead to massive leaks of personal data, seizure of control over user accounts, unauthorized financial transactions, or even paralysis of key operational processes. The consequences of a successful API attack can be just as, and sometimes more, severe than those of traditional web applications.

Finally, **Many developers and security professionals still lack sufficient knowledge and experience of specific threats and API security best practices **. Traditional approaches to application security are not always fully adequate. APIs require dedicated protection strategies that include strong authentication and authorization, precise input validation, protection against volumetric attacks, proper logging and monitoring, and regular, specialized security testing.

All of this makes APIs, those digital bridges that are often invisible to the end user, an extremely lucrative morsel for cybercriminals. Understanding these risks and taking proactive steps to secure them is an absolute necessity today for any organization that relies on modern digital technologies for its operations.

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What are the most common and insidious vulnerabilities lurking in your APIs according to the OWASP API Security Top 10?

As with web applications, the OWASP (Open Web Application Security Project) community provides invaluable guidance on the most serious security threats to APIs. The OWASP API Security Top 10 Project is a regularly updated list of critical vulnerabilities that any organization developing or using APIs should pay close attention to. Understanding these threats is the foundation for designing secure APIs and planning effective penetration testing.

Here are some of the key categories of vulnerabilities that frequently appear on the OWASP API Security Top 10 list (the specific points and their order may evolve in subsequent editions, but the overall risk areas remain similar):

Broken Object Level Authorization (BOLA) / Violated Object Level Authorization: This vulnerability, often considered one of the most serious, occurs when an API fails to properly verify that an authenticated user has the right to access a specific object (data record, resource) they are requesting. An attacker, knowing the identifier of an object belonging to another user, can try to access, modify or delete it, despite the lack of proper permissions. Example: user A can access user B’s order by merely changing the ID in the API request.
Broken Authentication: Weaknesses in API authentication mechanisms can allow attackers to take over the identities of legitimate users. This includes, but is not limited to, lack of protection against brute-force attacks on login points, poor management of session tokens (e.g., tokens that are too long-lived, predictable, transmitted in an unencrypted manner), or lack of or improper implementation of multi-factor authentication (MFA) for sensitive API operations.
Broken Object Property Level Authorization / Excessive Data Exposure: APIs often return more data than is actually needed by the client application, which then filters it on its side. An attacker, by analyzing API responses, can gain access to sensitive information that should not be visible to them, even if the user interface does not display it. Similarly, the API may allow modification of object properties that should not be accessible to the user in question.
Unrestricted Resource Consumption / Lack of Resources & Rate Limiting: If an API does not have adequate mechanisms to limit the number or size of requests it can process from a single client at a given time, it becomes vulnerable to Denial of Service (DoS) attacks or excessive resource consumption (CPU, memory, bandwidth). This can lead to slowdowns or complete unavailability of the service for legitimate users.
Broken Function Level Authorization (BFLA) / Violated Function Level Authorization: Similar to BOLA, but affects access to specific API functions or operations. Occurs when an API fails to properly verify that an authenticated user has the right to perform a specific action (e.g., a standard user can call a function intended only for an administrator). An attacker may try to directly call API endpoints corresponding to privileged functions.

Other common categories include: Mass Assignment (allowing the client to modify too many object fields at once, including those that should not be modified by the client), Security Misconfiguration (security configuration errors at the level of the API server, cloud platform, or libraries used), Injection (vulnerabilities such as SQL Injection, NoSQL Injection, Command Injection in API parameters), Improper Assets Management (such as. “shadow APIs” - undocumented or old versions of APIs that are not properly secured and monitored), and Insufficient Logging & Monitoring (lack of proper logging of security events and monitoring of API activity, making it difficult to detect attacks and respond to incidents).

Each of these vulnerabilities, if exploited, can lead to serious consequences. That’s why it’s so important that the process of designing, implementing and testing an API take into account these specific risks from the very beginning.

What are specialized API penetration tests and what unique techniques do ethical hackers use to test them?

API penetration testing, while sharing some fundamental principles with web application testing, requires a specialized approach, dedicated tools and an in-depth understanding of the unique attack vectors specific to this technology. Ethical hackers, playing the role of determined cybercriminals, employ a range of advanced techniques to uncover even the most hidden vulnerabilities in your digital bridges. It’s much more than just “clicking around the interface” - it’s precision reverse engineering and methodical investigation of every point of contact.

The process usually begins with a thorough understanding of the API documentation (if available) and mapping its attack surface. Pentesters analyze available endpoints, supported HTTP methods (GET, POST, PUT, DELETE, etc.), expected data formats (JSON, XML), authentication mechanisms (e.g., API keys, OAuth2/JWT tokens) and any publicly available information that can help understand the logic behind the API. Even if the documentation is incomplete or non-existent (a common problem with “shadow APIs”), experienced testers can reconstruct the API structure by analyzing the network traffic of client applications that use it, or through fuzzing techniques.

It then moves on to testing authentication and authorization mechanisms, which are absolutely key to API security. Among the things that are checked are:

Strength and security of access tokens: Are they long enough, are they random, do they have a limited lifetime, are they securely transmitted (HTTPS) and stored? Are they possible to be guessed, intercepted or forged?
Implementation of protocols such as OAuth2 or OpenID Connect: Are all flows (flows) implemented correctly? Are there no vulnerabilities to CSRF attacks in authorization flows? Are tokens verified correctly?
Testing object-level authentication (BOLA) and function-level authentication (BFLA): Once authenticated as a single user, pentesters attempt to access resources or functions belonging to other users or requiring higher privileges by manipulating object identifiers in requests or directly calling protected endpoints.

Another important area is the analysis and manipulation of input data. Pentesters meticulously test every parameter accepted by the API, looking for type vulnerabilities:

Injection: Attempts to inject malicious SQL, NoSQL, system commands (OS Command Injection) or XML code (XXE - XML External Entity) through crafted input data.
Mass Assignment: Checking whether, by submitting additional undocumented fields in a request, internal properties of an object that the user should not have access to can be modified.
Excessive Data Exposure: Analyzing API responses for sensitive information that is returned even though it is not used by the client application.

Specialized techniques also include testing the API’s business logic. Pentesters try to understand how the API works and what business processes it supports, and then look for errors in that logic that could be used for abuse (e.g., limit bypass, price manipulation, unauthorized execution of operations).

Don’t forget to test for resistance to volumetric attacks and resource management. It is checked whether the API has adequate mechanisms for limiting the number of requests (rate limiting) and the size of transmitted data to prevent DoS attacks or excessive load on servers.

These tests use tools familiar with web application testing (e.g. Burp Suite, OWASP ZAP - properly configured to work with APIs), as well as more specialized platforms and scripts dedicated to API testing (e.g. Postman with security testing add-ons, Kiterunner, Arjun). However, the key here is the knowledge and experience of the pentester, who can creatively combine these tools and techniques to uncover weaknesses invisible to automated scanners.

What fundamental principles of designing and implementing secure APIs (security by design) do you need to implement in your organization?

Truly effective API security is not something that can be “tacked on” at the end of the development process. It must be an integral part of the entire API lifecycle - from conception, through design and implementation, to deployment and maintenance. The “security by design” approach is a philosophy that sees security issues addressed at every stage, rather than treated as an add-on or a problem to be solved “later.” Implementing a few fundamental principles can significantly improve the security of your APIs.

1. strong authentication and authorization as the first line of defense:

Never trust input data: Every request to the API must be authenticated. Choose strong, standard authentication mechanisms (e.g., OAuth 2.0, OpenID Connect, API keys generated and managed securely). Avoid creating your own custom protocols.
Implement granular authorization: After authentication, carefully verify that the user (or client application) has permission to the requested resource and operation (BOLA and BFLA rules). Apply the principle of least privilege. Consider using role-based (RBAC) or attribute-based (ABAC) mechanisms.

2. Precise validation and cleaning of all input data:

Treat all data coming into the API as potentially malicious. Validate the format, type, length and range of all parameters. Reject requests that do not meet the defined criteria.
Use data sanitization (input sanitization) mechanisms to neutralize potentially dangerous characters or sequences that could lead to Injection attacks (SQLi, XSS, Command Injection). Use proven libraries and frameworks that offer built-in protection mechanisms.

3 Limiting data exposure and functionality:

Return only the data that is absolutely necessary for the client application. Avoid sending entire objects from the database if only selected fields are needed (data minimization principle).
Carefully define which object properties can be modified by which API endpoints (Mass Assignment protection).
Do not share unnecessary or sensitive information in response headers or error messages (e.g., detailed information about server version or database structure).

4. securing communications and protecting against volumetric attacks:

Always use TLS/SSL (HTTPS) encryption for all communication with the API to protect data in transit.
Implement rate limiting mechanisms at the user, API key or IP address level to protect APIs from DoS attacks and abuse.
Consider implementing a Web Application Firewall (WAF) or API Gateway with security features that can filter malicious traffic and protect against known attack vectors.

5. robust API lifecycle and documentation management:

Keep an accurate inventory of all your APIs (both internal and external), along with their versions and documentation. Avoid the creation of “shadow APIs” or “zombie APIs.”
Ensure that API documentation is always up-to-date, accurate and includes security information (e.g., required authentication mechanisms, expected data formats).
Implement a process for safely decommissioning old or unused versions of APIs.

6 Logging, monitoring and testing:

Implement detailed logging of all API requests and responses, as well as authentication, authorization and error events. These logs are crucial for attack detection and incident analysis.
Monitor API activity in real time for anomalies, suspicious traffic patterns or attempted attacks.
Regularly conduct specialized API penetration testing and vulnerability scanning to proactively identify and address security vulnerabilities.

Implementing these principles requires the involvement of both development teams and security professionals at every stage of the API lifecycle. It’s an investment that pays off in the form of more resilient, reliable and trustworthy interfaces that provide a solid foundation for your digital services.

How does nFlo help uncover and secure the “invisible door” of your APIs, ensuring the integrity and confidentiality of your data?

At nFlo, we fully understand that APIs, while often running in the background, are absolutely critical components of today’s IT architectures. They are like the “invisible doors and corridors” in your digital fortress - if not properly designed and secured, they can become an easy path for intruders, leading straight to your most valuable data and systems. That’s why we offer comprehensive services that help our clients not only discover these potentially hidden vulnerabilities, but more importantly, build robust and durable API protection mechanisms.

Our approach to API security is multidimensional and always starts with a deep understanding of your specific context. We analyze the architecture of your applications, how your APIs are used, the type of data you process, and the regulatory requirements that apply to you. We are not only interested in the technology, but also in the business processes that are supported by your interfaces. This allows us to tailor our activities to your real needs and priorities.

A key component of our offering is specialized API penetration testing. Our experienced ethical hackers, using the latest tools and techniques (including those described in the OWASP API Security Top 10), methodically examine every aspect of your APIs - from authentication and authorization mechanisms, to input validation, to business logic and protection against volumetric attacks. Our goal is not only to find individual vulnerabilities, but also to identify potential attack chains and assess the real risk to your organization.

After testing, we provide a detailed and understandable report that is not just a technical list. In it, we present a clear picture of the security status of your APIs, along with practical, prioritized recommendations for addressing identified vulnerabilities. We help you understand which problems need immediate attention and which can be addressed in the long term. Our recommendations range from changes to API code and configuration to suggestions for improving development processes (such as implementing Secure SDLC practices) or security policies.

However, our role does not end with auditing. At nFlo, we believe in building long-term solutions and competence on the client side. That’s why we also offer:

Support in designing secure APIs (Security by Design): We help your development teams incorporate security best practices from the very beginning when creating new interfaces.
Advice on the selection and implementation of appropriate protection technologies: Such as API Gateways, Web Application Firewalls (WAFs) with API protection functions, or systems for monitoring and analyzing API traffic.
Training for developers and testers: Raise their awareness and skills in secure API development and testing.
Assist in implementing continuous API security monitoring processes to ensure that new vulnerabilities are quickly detected and addressed.

With nFlo, you gain a partner that will not only help you “patch the holes” in your existing APIs, but more importantly, help you build a solid foundation and security culture that will ensure that your “invisible doors” are always properly protected, guaranteeing the integrity and confidentiality of your data and the continuity of your digital services.

Key findings: Security of APIs and Web Services

Aspect	Key information
API as a target for attacks	Invisible to the user but critical to application performance; “headless” nature makes traditional protection difficult; explosion in number and complexity of APIs; direct access to critical data and functions; often insufficient knowledge of specific API threats.
Most common API vulnerabilities (OWASP API Security Top 10)	Violated object-level authentication (BOLA) and function-level authentication (BFLA), violated authentication, excessive data exposure, unlimited resource consumption (no rate limiting), Mass Assignment, Security Misconfiguration, Injection, Improper Assets Management, Insufficient Logging & Monitoring.
Specialized API penetration testing	Understanding documentation and attack surface mapping, authentication and authorization testing (tokens, OAuth2, BOLA, BFLA), input analysis and manipulation (Injection, Mass Assignment), business logic testing, volumetric attack resiliency testing. Use of Burp Suite, OWASP ZAP, Postman, Kiterunner.
Fundamental principles of designing secure APIs (Security by Design)	Strong authentication and authorization, precise validation and cleansing of input data, limiting data exposure and functionality, securing communications (TLS/SSL) and protecting against volumetric attacks (rate limiting), robust API lifecycle and documentation management, logging, monitoring and regular testing.
nFlo’s support in securing APIs	In-depth understanding of customer context, specialized API penetration testing (OWASP compliant), detailed report with practical and prioritized recommendations, support in the design of secure APIs, advice in the selection of protective technologies (API Gateways, WAF), training for developers, assistance in the implementation of continuous monitoring.

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API and Web Services Security: How do you effectively protect the digital bridges that connect your applications and data?

Why have APIs, though often invisible to the user, become a favorite target of cybercriminals?

What are the most common and insidious vulnerabilities lurking in your APIs according to the OWASP API Security Top 10?

What are specialized API penetration tests and what unique techniques do ethical hackers use to test them?

What fundamental principles of designing and implementing secure APIs (security by design) do you need to implement in your organization?

How does nFlo help uncover and secure the “invisible door” of your APIs, ensuring the integrity and confidentiality of your data?

Key findings: Security of APIs and Web Services

Learn More

Explore Our Services

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Grzegorz Gnych

Want to Reduce IT Risk and Costs?

Why have APIs, though often invisible to the user, become a favorite target of cybercriminals?

What are the most common and insidious vulnerabilities lurking in your APIs according to the OWASP API Security Top 10?

What are specialized API penetration tests and what unique techniques do ethical hackers use to test them?

What fundamental principles of designing and implementing secure APIs (security by design) do you need to implement in your organization?

How does nFlo help uncover and secure the “invisible door” of your APIs, ensuring the integrity and confidentiality of your data?

Key findings: Security of APIs and Web Services

Related Terms

Learn More

Explore Our Services

Tags:

Share:

Talk to an expert

Grzegorz Gnych

Want to Reduce IT Risk and Costs?